The present invention relates to a novel cyanate ester based thermosetting resin composition with its moisture resistance improved markedly in addition to improvements in other properties such as electrical characteristics. The improved moisture resistance and low dielectric characteristics of this resin composition render it suitable for use as a molding material, a prepreg, a laminate, an adhesive, a structural material, a paint, a solvent-less liquid casting resin, and in many other applications as well.
The cyanate ester thermosetting resin composition (I) is known by cyanate ester resins (e.g. U.S. Pat. Nos. 3,553,244, 3,755,402, 3,740,348, and 4,578,439, and DEP 1,190,184 and 1,195,764), cyanate ester-maleimide resins and cyanate ester-maleimide-epoxy resins (e.g., U.S. Pat. Nos. 4,110,364 and DEP 2,512,085), and cyanate ester-epoxy resins (e.g., U.S. Pat. No. 3,562,214 and DEP 1,720,663).
These thermosetting resin compositions have superior characteristics such as high heat resistance, high chemical resistance, good mechanical and electrical properties, and high solder resistance. However, they suffer from the disadvantage of lower resistance to hot steam than epoxy resin compositions. Although they exhibit lower values of dielectric constant, .epsilon. and dissipation factor, tan .delta. than other thermosetting resins such as epoxy resins, still lower values are required to meet engineering standards that are becoming more and more strict these days.
The prior art cyanate ester based thermosetting resin compositions have been produced principally by one of the following methods: incorporating in cyanate ester thermosetting resins those low-molecular weight resins which are substantially free of functional groups, such as polystyrene, polyphenylene ether resins, and other thermoplastic resins having molecular weights of the order of 10.sup.3 ; or using cyanate ester thermosetting resins in combination with thermosetting resins having functional groups, such as polyfunctional maleimide resins, epoxy resins, diallyl phthalate resins, silicone resins, phenolic resins, phenol novolak resins, phenol-modified xylene resin, poly(acrylate), divinylbenzene and styrene. However, these methods have not proved to be completely satisfactory for the purpose of solving the aforementioned problem of poor resistance to hot steam compared with epoxy resin compositions.
It has been proposed that these known compositions be rendered flame-retardant by incorporating a variety of halogen-containing organic compounds, such as bromated polycarbonate oligomers having average degrees of polymerization of 2-15 or bromated epoxy resins. This method is effective in improving the flame retardancy of the compositions of interest but not in improving their resistance to hot steam and the best result that can be attained is that substantial deterioration of this property is avoided.
Conventionally, the properties of cyanate ester based resin compositions have been improved by either one of the following methods: they are cured or subjected to crosslinking reaction in the presence of compounds having functional groups that are reactive with the cyanato group; or in an attempt to provide ease of handling by allowing some deterioration of physical properties, the necessary physical properties are imparted by adding low-molecular weight resins (m.w. of the order of 10.sup.3) that are miscible at low temperatures. No technique has been available that is capable of achieving dramatic improvement in the resistance to hot steam of cyanate ester based resin compositions.